This invention concerns ejector mechanisms which are used to eject formed parts from tooling used in their manufacture. In thermoforming parts, a well known process is to vacuum form an array of cups into a single sheet of plastic. The formed sheet is then transferred into a trim die where the cups are cut free from the rest of the sheet and ejected into a stacking mechanism and transferred as a stack for further processing.
The ejector comprises an array of pins held on a member, the pins advanced into the parts after die cutting is complete the same from the sheet to push the cup free of the trim die. Such ejection in the past has been typically accomplished by air cylinders cycling the pin motion. Production speeds have greatly increased in recent years, and reversing servo motors driving slider crank mechanical movements have been used to increase the ejector cycle rate. In this arrangement, the servo motor drives a crank which has an arm pinned to one end of a link, the link pinned at the other end to a slider mounting the ejector pins.
The servo motor rotates the crank to advance the slider to a maximum advance or eject position, reverses, and then retracts and reverses again to begin the next cycle. Thus, each cycle requires two reversals of the servo motor.
This arrangement has worked satisfactorily despite the need for two servo motor reversals, and offered each changes in the retract and advanced positions to adapt to tooling changes. However, because the demand for even higher cycle rates has grown, this arrangement has been found to be deficient as being not capable of executing even more rapid cycle rates now desired, primarily due to the need to reverse the motor twice for each ejection cycle.
It is an object of the present invention to provide a part ejector of the type operated by a reversible servo motor which is capable of higher cycle rates than the prior reversible servo motor driven part ejectors.
The above recited objects as well as other objects which will become apparent upon a reading of the following specification and claims are achieved by a method for driving part ejectors in which only a single reversal of the servo motor is required per ejector cycle. This is achieved by continuing to advance the servo motor unidirectionally past the point at which the ejector reaches its maximum travel at the crank dead center position, retracting the ejector pins by continued rotational advance of the servo motor in the same direction, the crank-slider causing retraction of the slider to a retracted position, where the servo motor is reversed preparatory to the next cycle. Thus, only a single reversal of the servo motor is required per ejector cycle.